Bluetongue virus transmission between ruminant host and Culicoides vectors: the importance of insect saliva proteins

蓝舌病毒(Bluetongue Virus，BTV)是一种由节肢动物传播的病毒，是一种虫媒病毒，通过摄食蚊虫(Culicoidesspp.)的血液在反刍动物宿主之间传播。该病毒可以感染大多数反刍动物，但会在绵羊(特别是欧洲品种)、一些种类的鹿中引起出血热和严重的临床疾病，在牛中的感染程度较小。直到十年前，蓝舌病还是一种温暖气候的疾病，那里有传播能力强的昆虫媒介物种的种群。然而，自1998年以来，BTV已逐步向北进入欧洲，这一过程与当地气候变暖和某些蚊媒种群的扩大分布有关。2006年，一场蓝舌疫情比以往任何时候都发生在更靠北的地方，影响到比利时、荷兰、德国、卢森堡和法国东北部。该病毒在羊甚至牛身上引起严重的临床疾病，大约50%的羊和10%的牛出现临床症状而死亡。这次暴发证实了一些北欧蚊种(与英国相同)可以传播BTV，并表明包括英国在内的整个欧洲现在正面临进一步暴发的威胁。然而，并不是所有种类的蚊子都能有效地传播BTV，这表明病毒的传播和疾病的分布受到涉及的昆虫媒介和病毒株的影响。食血昆虫将唾液沉积到宿主的皮肤中。这会导致皮肤发炎和过敏，在蚊子或蚊子叮咬后，会导致常见的红斑和瘙痒，甚至是过敏性水泡。BTV在昆虫的唾液中传播，必须在炎症反应中幸存下来，才能感染反刍动物宿主。然而，唾液也含有蛋白酶(帮助防止凝结)，可以修改病毒，甚至增加其对昆虫媒介的传染性。人们通常认为虫媒病毒是通过喂养昆虫进入宿主的血液中的。然而，蚊子使用它们的口器来切割和破坏皮肤组织，这样它们就可以以血液、组织和淋巴液的混合池为食。BTV不是注射到血管中，而是留在唾液中沉积到皮肤中。病毒可以在某些皮肤细胞中复制，此外还可以在被叮咬部位吸引的炎症反应宿主细胞中复制。因此，BTV在皮肤中的含量可能高于血液中的含量，这可能对喂养蚊子的摄取很重要。因此，这种病毒似乎已经进化到利用唾液蛋白和宿主的反应，以增加传播效率和生存机会。我们最近开发了大量分离蚊虫唾液蛋白的方法，以研究它们在媒介昆虫和反刍动物宿主之间传播BTV中的作用。初步研究表明，不同种类的蚊子有不同的唾液蛋白，这可能有助于解释它们媒介“能力”的差异。皮尔布赖特有独特的设施来研究蚊子、绵羊和蓝舌病病毒本身，这将使我们能够探索它们之间的关系和对传播机制的影响。该项目将比较不同欧洲、非洲和美洲蚊子的唾液，以确定唾液蛋白在哺乳动物和昆虫组织培养细胞或绵羊BTV感染中的作用。该项目将阐明皮肤中本地产生的病毒对BTV传播的重要性，并将确定病毒是否可以在感染和未感染的绵羊之间机械传播(通过其他昆虫或通过针头)，或在以同一只绵羊为食的同时从一只蚊子直接传播到另一只蚊子。这项研究的结果将提高我们对BTV在欧洲传播的了解，有助于确定蓝舌传播的风险并实施适当的控制措施。

Bluetongue virus (BTV) is an arthropod-borne virus (an Arbovirus) transmitted between its ruminant hosts by blood feeding 'biting' midges (Culicoides spp.). The virus can infect most ruminants but causes haemorrhagic fever and severe clinical disease, in sheep (especially European breeds), some species of deer and to a lesser extent in cattle. Until a decade ago bluetongue was a disease of warm climates, areas containing populations of transmission-competent insect vector species. However, since 1998 BTV has moved progressively further north into Europe, a process related to warming of the local climate and an expanding distribution of certain midge vector populations. In 2006, a bluetongue outbreak occurred much further north than ever before, affecting Belgium, the Netherlands, Germany, Luxembourg, and North East France. The virus causes serve clinical disease in sheep and even cattle and approximately 50% of sheep and 10% of cattle developing clinical signs die. This outbreak confirms that some northern European midge species (which are the same as in the UK) can transmit BTV, and demonstrates that the whole of Europe, including the UK, is now under threat of further outbreaks. However, not all species of midge can transmit BTV efficiently, suggesting that transmission of the virus and the distribution of the disease is influenced by the insect vectors and the strain of virus involved. Blood feeding insects deposit saliva into the skin of the host. This causes inflammation and hypersensitivity in the skin, which results in the familiar red spots and itching, or even allergic vesicles, after midge or mosquito bites. BTV is transmitted in the insect's saliva and must survive the inflammation response in order to infect the ruminant host. However, the saliva also contains proteases (to help prevent clotting) which can modify the virus, even increasing its infectivity for the insect vector. It is often assumed that Arboviruses are injected into the blood of the host by feeding insects. However, midges use their mouthparts to cut and damage skin tissues, so they can feed on a mixed pool of blood, tissue and lymphatic fluid. BTV is not injected into blood vessels but left in the saliva deposited into the skin. The virus can replicate in certain skin cells and additionally in cells of the hosts inflammatory response that are attracted to the biting site. Therefore BTV may be present at higher levels in the skin than in the bloodstream, which may be important for uptake by feeding midges. So it appears that the virus has evolved to make use of the saliva proteins and the host's response, to increase the efficiency of transmission, and its chances of survival. We have recently developed methods to isolate midge saliva proteins in large quantities, to investigate their role in BTV transmission between vector insects and ruminant hosts. Initial studies indicate that different midge species have different saliva proteins, which may help explain differences in their vector 'competence'. There are unique facilities at Pirbright to work on midges, sheep and the bluetongue virus itself, that will allow us to explore their relationships and influence on transmission mechanisms. The project will compare saliva from different European, African and American midges, to determine the role of saliva proteins in BTV infection in mammalian and insect tissue culture cells, or in the sheep. The project will elucidate the importance of locally generated virus in the skin for BTV transmission and will also determine if the virus can be transmitted mechanically between infected and non-infected sheep (by other insects or by needle), or directly from one midge to another while they feed on the same sheep. The results of this research will improve our understanding of BTV transmission in Europe, helping to define the risk of bluetongue transmission and to implement appropriate control measures.